Recently, manufacture of sensors or actuators, with microscopic dimensions, and by using micromachining, is attracting attention.
For example, a semiconductor sensor with a three-dimensional structure having a cross-section shown in FIG. 1 has been proposed. In FIG. 1, for example, a microscopic space 3 giving a beam interval of 10 μm and a floating height of 2 μm from the silicon chip 1 is created in a silicon chip to form a cantilever 2. This microscopic space 3 is created during the formation of a silicon substrate 4 by embedding a sacrificial oxide film layer 5 into the portion which becomes the space later, forming a sensor structure on a silicon layer 6 above the sacrificial oxide film layer 5 by patterning, selectively oxidizing the silicon layer 6, and then dissolving and removing the oxide pattern 7 between beams 2 and the sacrificial oxide film layer 5 in the portion under the beam 2. To remove this oxide film layer, removal by a hydrofluoric acid solution is the simplest and easiest method but when the oxide film layer becomes a winding microscopic space and is treated with a hydrofluoric acid solution as a liquid, the liquid attaches to the narrow space during the removal treatment or the drying of the treating solution and, by the surface tension thereof, a movable beam is drawn to a fixed beam. When drying is performed when this force is applied, the movable beam sticks to the fixed beam in a stuck state and the product obtained cannot be used as a sensor. Therefore, in order to attain the removal while not imposing a load such as surface tension of liquid, use of a gaseous hydrofluoric acid (hydrofluoric acid anhydride) is necessary. However, the reaction proceeds at an extremely low rate with only a hydrofluoric acid anhydride and therefore, water such as pure water or an alcohol must to be mixed in the form of a vapor. More specifically, as shown in FIG. 3, a silicon chip 1 comprising a sacrificial oxide film layer 5 is placed in a treatment chamber 11 and a hydrofluoric acid anhydride 13 and water vapor 14 are introduced into the treatment chamber 11 under heating by a heating device 12 and mixed in the treatment chamber 11, thereby removing the sacrificial oxide film layer 5.
However, the method of introducing a hydrofluoric acid anhydride 13 and a water vapor 14 into the above-described treatment chamber 11 and mixing them in the treatment chamber 11 has a problem in that the balance between the hydrofluoric acid anhydride and the water vapor is difficult to control and, in the case of processing a complicated structure, the microscopic space is filled with excess water and, after the removal of the sacrificial oxide film layer, the structure collapses.
If the amount of water vapor is reduced not to generate excess water, this processing method is difficult to use in practice because, for example, the reaction rate seriously decreases and the process becomes extremely low in the productivity.
Under these circumstances in conventional techniques, an object of the present invention is to supply microfine liquid droplets to the above-described microscopic space and, to realize this, to provide a method and an apparatus for forming microfine liquid droplets.